Relax. It's almost certainly not the diff. I can't recall ever reading about a failed 123 differential.
It's an axle shaft. The intermittent nature of the sound is a classic symptom. It's probably the one with the likely torn boot. If it's really the original, I'd say you got pretty good service out of it.
Not a hard job to replace--no special tools other than maybe a deep socket to get the screw that holds the shaft to the hub.
Do not replace with a cheap Chinese axle. I recommend CVJ out of Denver.
CVJ Axles - CV Axles & Steering Racks for all your needs